Since the radar level gauging was developed as a commercial product in the 1970's and 1980's, frequency modulated continuous wave (FMCW) has been the dominating measuring principle for high accuracy applications. An FMCW measurement comprises transmitting into the tank a signal which is swept over a frequency range in the order of a few GHz. For example, the signal can be in the range 25-27 GHz, or 9.5-11 GHz. The transmitted signal is reflected by the surface of the contents in the tank (or by any other impedance transition) and an echo signal, which has been delayed a certain time, is returned to the gauge. The echo signal is mixed with the transmitted signal to generate a mixer signal, having a frequency equal to the frequency change of the transmitted signal that has taken place during the time delay. Due to the linear sweep, this difference frequency, also referred to as an intermediate frequency (IF), is proportional to the distance to the reflecting surface. The mixer signal is often referred to as an IF signal.
Although highly accurate, present FMCW systems are relatively power hungry, making them less suitable for applications where power is limited. Examples of such applications include field devices powered by a two-wire interface, such as a 4-20 mA loop, and wireless devices powered by an internal power source (e.g. a battery or a solar cell). Various methods are available to decrease power consumption.
In U.S. Ser. No. 12/981,995, by the same inventor, a novel and less power hungry measuring principle was introduced, involving emitting a series of pulses having constant carrier wave frequency, each pulse being long compared to the time of transit (e.g. a pulse duration in the order of 10 us to 100 ms, compared to time of transit in the order of tenths of a μs). The method is therefore referred to as a Multiple Frequency Pulsed Wave (MFPW).
The number of different carrier wave frequencies in a measurement cycle is insufficient to provide a continuous IF signal, or even an approximation of the IF frequency in the way done in so called “stepped” or “discrete” FMCW system. Instead, the small set of frequencies is chosen according to a specified frequency scheme, and a phase shift in the received pulse is determined for each frequency.
The process of determining the distance to the surface involves establishing a change of phase with emitted frequency (see FIG. 1). The line A represents an initial distance estimation, while line B represents an updated estimation. In theory, only two values (points x) are required to determine the rate of change (slope of line B), while in practice a larger number, e.g. a 5-20 samples, may be required. During a more complicated echo situation or during start-up a few hundred may be required. Naturally, from a power economy point of view, it is desirable to provide high accuracy and reliability with as few samples as possible.
One way to increase accuracy without requiring many samples is to determine a “rough approximation” of the distance to the surface, and to use this estimate as a starting point when determining an accurate distance measure.
General Disclosure of the Invention
Based on the above, it is an object of the present invention to provide an improved radar level gauging system of the kind with pulses of constant frequency which are long compared to the time of transit, which system is capable to provide a rough distance estimate independently of the existing measurement.
According to a first aspect of the present invention, this and other objects are achieved by a method for determining a distance to a surface of a product kept in a tank, the method comprising forming a transmit signal as a pulse train of distinct carrier wave pulses having a duration greater than 1 microsecond and shorter than 100 milliseconds, each pulse being frequency modulated around a defined center frequency, transmitting the transmit signal towards the surface, receiving an electromagnetic return signal reflected at the surface, mixing the return signal with the transmit signal in a first channel (I-channel) and mixing the return signal with a 90° phase shifted transmit signal in a second channel (Q-channel), to provide two IF (intermediate frequency) signals, filtering the IF signals to provide a filtered signal corresponding to a first selected harmonic of the modulation frequency, mixing the filtered signals of each channel with the first selected harmonic of the modulation frequency, to provide two primary amplitude values (I and Q), calculating (step S5) actual phase properties of each distinct pulse received in relation to each corresponding distinct pulse transmitted, based on the primary amplitude values and determining a distance measure based on the actual phase properties, filtering the IF signals to provide a filtered signal corresponding to a second selected harmonic of the modulation frequency, mixing the filtered signals of each channel with the second selected harmonic of the modulation frequency, to provide two secondary amplitude values (I and Q), and providing an approximation of the distance based on a relationship between the primary and the secondary amplitude values.
According to a second aspect of the present invention, this and other objects are achieved by a radar level gauging system, for determining a distance to a surface of a product kept in a tank, the system comprising a transceiver for transmitting electromagnetic transmit signals formed by a pulse train of distinct carrier wave pulses having a duration greater than 1 microsecond and shorter than 100 milliseconds, each pulse being frequency modulated around a defined center frequency, and receiving electromagnetic return signals reflected at the surface, a set of RF mixers for mixing the return signal with the transmit signal in a first channel (I-channel) and mixing the return signal with a 90° phase shifted transmit signal in a second channel (O-channel), to provide two IF (intermediate frequency) signals, a first set of filters for filtering the IF signals to provide a filtered signal corresponding to a first selected harmonic of the modulation frequency, a first set of IF mixers for mixing the filtered signals of each channel with the first selected harmonic of the modulation frequency, to provide two primary amplitude values (1 and Q), a second set of filters for filtering the IF signals to provide a filtered signal corresponding to a second selected harmonic of the modulation frequency, a second set of IF mixers for mixing the filtered signals of each channel with the second selected harmonic of the modulation frequency, to provide two secondary amplitude values (I and Q), and processing circuitry connected to receive outputs from the IF mixers and operable to calculate actual phase properties of each distinct pulse received in relation to each corresponding distinct pulse transmitted, based on the primary amplitude values and determining a distance measure based on the actual phase properties, and providing an approximation of the distance based on a relationship between the primary and the secondary amplitude values.
The level gauging thus includes an approximation of a distance to the surface. The approximation is determined by relating an amplitude of a first harmonic of an IF signal with an amplitude of a second harmonic of the IF signal.
The frequency modulation of the transmit signal introduces a distance dependence. Two different harmonics of the modulated signal are used in two separate signal paths (sequential or parallel) to provide two different values representing different distance dependencies. By comparing these two values, e.g. dividing one with the other and forming a quotient between them, a rough approximation of the distance to the surface can be provided.
Basically, each harmonics represents a given distance range. By determining the received power in two or more harmonics, and correlating them to each other, the distance may be estimated. Depending on modulation and other parameters the distance dependence may be very different, and may be selected to suit the application.
The actual phase of the IF-signal of a plurality of pulses may be used in the process of providing a more exact distance measure, e.g. as disclosed in U.S. application Ser. No. 12/981,995, herewith incorporated by reference.
According to one approach, the approximated distance is used as input in the accurate distance determination, e.g. to improve the statistical analysis applied to a plurality of phase properties, thereby enabling a more sensitive and reliable distance measurement. Alternatively, the rough approximation can be used as a post processing verification of the accurate measurement. If the rough approximation is significantly different than the accurate distance measure, then a new accurate distance measurement should be obtained.